Here is the Valve 'DOTA' Trademark on USPTO:
Mark : 'DOTA'
Applicant on Record: Valve Corporation
10500 NE 8th St Ste 1000
Bellevue, WA 980044359
Legal Entity Type: Corporation
State or Country of Incorporation: Washington
Phone Number: 509.241.1502
Fax Number: 509.232.2112
Date applied: August 10 2010
International Class: 009
Class Status: Active
Computer game software; Electronic game software; Video game software
Let's see if Valve gets Trademark for 'DOTA'. In my opinion it will be hard for Valve to pick up existing public domain material and trademarking it. DotA is the product developed by Euls, Guinsoo, IceFrog and by the help of community suggestions. Also, Valve has to make sure if the are putting content with Blizzard's Warcraft III Trademark on their DotA based game.
Things are getting really interesting in DotA Genre games, what about Heroes Of Newerth, League, Bloodline Champions, Demigod etc. Are they taking note?